1. Field of the Invention
The present invention relates to the floor panel structure of a vehicle body and particularly to the floor panel structure of a vehicle body where the floor of an vehicle has floor panels linked to a plurality of frame members that are disposed in the vehicle body lengthwise direction and the vehicle body crosswise direction and linked to an engine or a suspension.
2. Conventional Art
Vibration from frame members linked to the engine or suspension is known to be transmitted to floor panels, causing these floor panels to vibrate and as a result, the air within the passenger cabin vibrates greatly, thus generating unpleasant in-cabin vibrations and noise.
In this case, the source of vibration causing the problem may be vibration from the engine itself or road noise transmitted from the suspension, while the road noise typically includes components due to resonance of the tire cavity and components due to resonance of the suspension.
Typical measures conventionally taken to suppress the vibration and noise include applying vibration-damping materials and vibration-suppressing materials as various vibration-damping and vibration suppression measures. While it is possible to reduce vibration and noise in this manner, an extremely large amount of vibration-damping material and vibration-suppressing material is required, thus increasing the vehicle weight and leading to various deleterious effects and becoming a major problem on the cost side.
Moreover, the unpleasant vibration transmitted from the engine and suspension is mainly below 400 Hz in a vehicle, and in particular, has a peak near 250 Hz, which is the frequency of road noise arising from tire cavity resonance. Thus, a technique is known by which a plurality of beads is formed in the floor panels, thus increasing the panel thickness and raising its rigidity, thereby shifting the natural frequency of the floor panel to a high band higher than 400 Hz. In other words, an attempt is made to prevent the floor panel from resonating at the resonance frequency of the suspension and the tire cavity resonance frequency band, thus reducing unpleasant vibration and noise.
In this case, while this has the advantage of being able to suppress resonance peaks in low-frequency regions, vibration in the high-pitched regions conversely increases, so it becomes necessary to use large amounts of vibration-damping materials and vibration-suppressing materials in order to suppress vibration and noise in the high-frequency regions. In this manner, even in this case, the vehicle weight is increased as described above so there are various deleterious effects and problems on the cost side, so it is desirable to solve this problem.
Thus, the present inventors focused on the relationship between the frequencies and vibration modes of vibrations transmitted to the floor and proposed a structure of a floor panel that has vibration modes wherein the acoustic emission level is smaller at specific vibration frequencies (resonance regions) (see Japanese Patent Unexamined Publication No. JP-A-9-202269). The floor panel structure is one wherein the specific frequencies are frequencies near the 250 Hz of road Fnoise arising from the tire cavity resonance transmitted to the floor panel as the most unpleasant vibration, and so the rigidity of the floor panel is partially adjusted so that the vibration mode of the floor panel becomes a vibration mode such as a 2×2 mode or 2×1 mode where an even number of vibration antinodes is generated, and thus with a setup where the sound waves radiated from the respective vibration antinodes cancel each other, it is possible to reduce the acoustic emission level and reduce noise within the cabin.
However, in the case in which vibration-damping materials and vibration-suppressing materials are attached to the entire surface of the floor panel as described above, there are problems of increased materials costs and increased vehicle weight. In addition, if the panel thickness is increased, there is also a problem of increased vehicle weight.
In addition, with the floor panel structure recited in JP-A-9-202269 above, while effective in reducing the noise in specific frequency regions, it is difficult to reduce noise in frequencies outside those specific frequency regions at the same time, so there is a problem in that in order to reduce vibration in a wide range of frequency bands at the same time, it is necessary to apply vibration-suppressing material to the entire surface of the floor panel, thus increasing the vehicle weight.
Here, the present inventors took note of the fact that vibration from the engine and vibration from the suspension (road noise), which are the main causes of the most unpleasant in-cabin vibrations and noise, are transmitted to the floor panel via the frame members of the body and that their vibration damping effect is greater the larger the deformation of the vibration-suppressing material and thus attempted to solve the aforementioned problems with the conventional art.